Biodegradable plastics are plastics that decompose in the natural environment. That is, upon exposure to microorganisms in the natural environment (e.g., soil), biodegradable plastics are metabolized and their molecular structure is broken down. Biodegradability in plastics is desirable because such plastics may be diverted from landfill sites. When disposed in microbially rich environments such as active compost and activated sludge, biodegradable plastics rapidly convert to nitrogen-, phosphorus-, and carbon-containing components that can be taken up by organisms such as plants. In contrast to non-biodegradable plastics, which are mostly produced from non-renewable resources such as petroleum and hydrocarbon raw materials, most biodegradable plastics are produced from renewable natural sources and provide sustainable technology. Markets for biodegradable plastics include, for example, products that end up in sewage systems and marine environments.
A particular type of biodegradable plastic is a family of polyesters called “polyhydroxyalkanoate” or simply “PHA”. Besides the advantages in biodegradability, biocompatibility, and renewability compared to petrochemical plastics, PHAs also offer unique advantages such as resistance to water hydrolysis and low gas permeability when compared to other types of biodegradable plastics such as polylactides and starch-derived plastics.
Polyhydroxyalkanoates (PHAs) are polyesters synthesized and accumulated by many microorganisms. They are considered a major class of biopolymers and have attracted extensive research and industry interests in recently years. Their properties resemble those of many petroleum-based plastics, while offering inherent biodegradability and biocompatibility. However, unlike most plastics, PHAs are produced from renewable resources such as sugars and plant oils.
In the PHA polymer family there are two main types of polymers, short-chain-length poly(3-hydroxyalkanoate) (SCL-PHA) and medium-chain-length poly(3-hydroxyalkanoate) (MCL-PHA). SCL-PHAs are classified as having 3 to 5 carbons in their repeating units. MCL-PHAs are classified as having more than 5 carbons in the repeating units. Characteristics such as crystallinity and melting points differ between SCL-PHAs and MCL-PHAs. Accordingly, SCL-PHAs are better suited for certain applications while for others, MCL-PHAs are better suited.
There are numerous microorganisms that are able to synthesize SCL-PHAs such as poly(3-hydroxybutyrate) (PHB) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV). Cupriavidus necator, formerly known as Ralstonia eutropha, is the most studied SCL-PHA-producing bacterium. SCL-PHA production by SCL-PHA-synthesizing bacteria has been well studied and it has been determined that nitrogen (N) or phosphate (P) limitation often stimulates rapid SCL-PHA synthesis. Thus, most SCL-PHA production processes employ a stage of rapid cell growth followed by a SCL-PHA accumulation stage, which is almost always N-limited or P-limited. Possibly due to an assumption that physiology of MCL-PHA-accumulating bacteria is similar to that of most SCL-PHA-accumulating bacteria, almost all publications dealing with MCL-PHA production have incorporated N or P limitation. While for certain bacterial strains, MCL-PHA production rates are stimulated by N-limitation, other strains have no improvement with N- or P-limitation (see Sun, Z. et al. (2007) Appl. Microbiol. Biotechnol. 74: 69-77).
As for naturally occurring MCL-PHA-producing microorganisms, MCL-PHAs have only been found in pseudomonads belonging to the rRNA homology group I, such as Pseudomonas fluorescens and Pseudomonas putida (Diard, S., et al. (2002) Syst. Appl. Microbiol. 25: 183-188).
In contrast to SCL-PHAs, MCL-PHAs are generally thermo-elastomeric materials with low melting temperature and low degree of crystallinity. Their use as biodegradable adhesives, rubbers, coatings, tissue engineering scaffolds, controlled drug delivery carriers, and toner agents has been proposed. However, due to the lack of an efficient production method for a variety of MCL-PHAs, these polyesters are not yet well studied. In particular, since under previously known technology there is no method to reliably produce MCL-PHAs with certain selected ratios of monomers, there is a lack of research as to the applications of such polymers. Thus there are unexplored avenues of new polymers. Indeed, since changing the monomeric composition of MCL-PHAs is expected to result in changes to properties of the material, there are also unexplored new uses of such new polymers. Therefore, there is a need for a method of controlling production of monomeric composition of MCL-PHA.